Levels 5–8: Intermediate

Level 5: Process Management

Linux runs hundreds of processes at once. You need to control them.

Command	What it does
`ps aux`	List ALL running processes (all users)
`ps aux \| grep nginx`	Find a process by name
`kill PID`	Send SIGTERM (polite stop) to process
`kill -9 PID`	Send SIGKILL (force stop, cannot be ignored)
`jobs`	List background jobs in current shell
`bg`	Resume a stopped job in the background
`fg`	Bring background job to foreground
`command &`	Start a command directly in the background
`top`	Live process viewer (q to quit)
`lsof -i :80`	Show what process is using port 80

ps aux columns:

Column	Meaning
`USER`	Owner of the process
`PID`	Process ID
`%CPU`	CPU usage
`%MEM`	Memory usage
`STAT`	State: R=running, S=sleeping, Z=zombie, T=stopped
`COMMAND`	The command that was run

Transform data at machine speed without a GUI.

Command	What it does	Example
`cut -d: -f1 file`	Extract field 1, delimiter :	`cut -d: -f1 /etc/passwd`
`awk '{print $1}' file`	Print first field of each line	Column extraction
`awk '$2 > 50' file`	Filter rows by condition	Numeric comparisons
`awk 'END{print sum}' file`	Run code after all lines	Totals and summaries
`sed 's/old/new/g' file`	Replace all occurrences	Find and replace
`sed '/pattern/d' file`	Delete lines matching pattern	Strip noise
`sed -n '2,5p' file`	Print lines 2–5 only	Extract a range
`head -n 10 file`	First 10 lines	Preview
`tail -n 10 file`	Last 10 lines	Recent entries
`tr 'a-z' 'A-Z'`	Translate/convert characters	Case conversion

awk built-in variables:

Variable	Meaning
`$0`	Entire line
`$1`, `$2`…	Fields 1, 2…
`$NF`	Last field
`NR`	Current line number
`NF`	Number of fields on current line
`FS`	Field separator (default: whitespace)

Linux is the internet. Know how to interact with it.

Command	What it does
`ping -c 4 host`	Test connectivity, 4 packets
`curl url`	HTTP GET request, print response
`curl -o file url`	Download URL to file
`curl -X POST -H 'Content-Type: application/json' -d '{...}' url`	POST JSON
`wget url`	Download file (saves automatically)
`ssh user@host`	Connect to remote server
`ssh -i ~/.ssh/key user@host`	Connect with specific key
`scp file user@host:/path/`	Copy file to remote server
`ss -tlnp`	Show listening TCP ports with process names
`netstat -tlnp`	Same (older tool, still widely available)

curl common flags:

Flag	Meaning
`-o file`	Save output to file
`-O`	Save with remote filename
`-L`	Follow redirects
`-s`	Silent mode (no progress)
`-I`	HEAD request only (headers)
`-u user:pass`	Basic authentication
`-k`	Skip SSL certificate verification

Automate everything. Scripts are commands in a file.

#!/bin/bash
# Every script starts with a shebang: tells the OS which interpreter to use

NAME="BashQuest"
echo "Hello from $NAME"

File test operators: